Recently, as the adoption of broadband technology in the communication infrastructure proceeds in an explosive pace and the information processing capacity of computers and other hardware increases at a dramatic pace, there are increasing needs for an information processing circuit that has a very fast information transmission line. Thus as a means for breaking through the limit of transmission speed of electrical signals, transmission of information in the form of optical signals has been studied and various attempts have been made on the technology to mount an optical circuit on a substrate that has an electrical circuit formed thereon.
The basic idea of mounting the electrical circuit and the optical circuit together is to form in addition to the existing electrical circuit the optical circuit on the conventional printed circuit board. For the manufacture of the optical circuit-electrical circuit mixedly mounting substrate where the optical circuit and the electrical circuit are stacked in multiple layers, the following two methods have mainly been proposed:
With one method, a cladding layer, a core layer and a cladding layer that constitute an optical waveguide of the optical circuit are stacked one on another on a substrate whereon an electrical circuit has been formed, and an electrical wiring layer is further formed thereon by plating or the like.
With the other method, a cladding layer, a core layer and a cladding layer are stacked one on another on a dummy circuit board to form an optical waveguide that constitutes an optical circuit, then the optical waveguide is bonded onto a printed circuit board followed by removing the dummy circuit board, and an electrical circuit is formed on the optical waveguide by plating or the like. Reference may be made for this method to, for example, Japanese Unexamined Patent Publication No. 2001-15889.
The methods mentioned above have such problems that a number of steps is large which are required for stacking by forming the optical circuit and the electrical circuit successively. The accuracy of wiring is low when forming the electrical circuit by plating as is the common practice in the prior art. Thus it is difficult to manufacture high-quality optical circuit-electrical circuit mixedly mounting substrates stably on a industrial basis.